Sorry for bringing up an old thread, and I don't know if the author still monitors, but I was curious about the parallax issue in the 'aligning the red dot' section of the report. I understand that the red dot on the laser gun appears in a head-up display and through the clever use of optics appears much further away. Clearly this would be necessary otherwise the dot would appear out of focus to the operator and impossible to view (being a few cm away) at the same time as the target vehicle (possibly hundreds of metres away). Is it therefore the case that that diagram in the report should have the 'sight' position much higher therefore producing triangles which are much more elongated? Furthermore, in the example given if the red-dot is optically a few hundred metres away then how does this affect the target error?

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"Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things" - Isaac Newton

Sorry for bringing up an old thread, and I don't know if the author still monitors, but I was curious about the parallax issue in the 'aligning the red dot' section of the report. I understand that the red dot on the laser gun appears in a head-up display and through the clever use of optics appears much further away. Clearly this would be necessary otherwise the dot would appear out of focus to the operator and impossible to view (being a few cm away) at the same time as the target vehicle (possibly hundreds of metres away). Is it therefore the case that that diagram in the report should have the 'sight' position much higher therefore producing triangles which are much more elongated? Furthermore, in the example given if the red-dot is optically a few hundred metres away then how does this affect the target error?

No.

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Andy

If you're going to try to contradict me, please at least try to get your facts straight.

Sorry for bringing up an old thread, and I don't know if the author still monitors, but I was curious about the parallax issue in the 'aligning the red dot' section of the report. I understand that the red dot on the laser gun appears in a head-up display and through the clever use of optics appears much further away. Clearly this would be necessary otherwise the dot would appear out of focus to the operator and impossible to view (being a few cm away) at the same time as the target vehicle (possibly hundreds of metres away). Is it therefore the case that that diagram in the report should have the 'sight' position much higher therefore producing triangles which are much more elongated? Furthermore, in the example given if the red-dot is optically a few hundred metres away then how does this affect the target error?

No.

Thanks for the concise answer. Can you please expand to explain why when looking through the scope, and the red dot is in sharp focus and appears to be much further away, there is not much apparent movement in its position on the target vehicle when your eye position moves?

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"Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things" - Isaac Newton

If you look through a telescopic sight the cross-wires are in sharp focus, and their position relative to the target doesn't change if you move your eye away from the centre line. I don't know how that's done optically, but presumably the same may be done with this speed gun.

I am consulting on a case using a Ultralte LTI LR100. I a trying to gather as much information as possible on this device. Unfortunately, my request are rather precise. Hence, the local distributor (I am living in Quebec) can't answer and the company refuse to answer.

Maybe someone here can answer my question.

Is the field of view is limited by the detector of the laser beam. If it is the laser beam what is its half power beamdwidth. This may look symmetrical but behaviour is different.

Does anyone know why the uncertainty is 1 MPH for a device that can measure distance to within 6 in. This look a simple limitation of the display, but the conversion in metric is 2 km/h, which eitehr say the truth or a demonstration of the incompetency of the analysis.

Third, what is the official claim about how the anti-sweep error algorithm works. I have got different version of it. IMHO the parameters are relaxed in difficult operating conditions (long distance, small target, non perpendicular target, large road reflection) leading to erroneous measurements.

I will try to build a full model of the instrument including atmospheric propagation. Off course, more I know before, the best it is.

As you will know the accuracy of the device will far exceed +/- 1mph (or equivalent rounded km/h) but one would guess for simplicity that the display only displays integers so with rounding (down) and the possible cosine reduction due to a non-perpendicular measurement then the manufacturer would be a hostage to fortune in listing a more accurate headline accuracy.

In the UK we have quite a generous threshold before enforcement anyway. Such accuracy makes no difference whatsoever in the real world - I don't see any lines of 'attack' here.

I don't know how Canadian law works but here the devices are 'Type Approved' and the measurement is assumed correct unless the defendant can show otherwise - which in reality is near impossible. (But of course the reading is more likely than not to be correct). Additionally, the enforcement officer should be using the device to corroborate their opinion - in most circumstances it's easy to spot a speeding motorist.

The 'anti-sweeping' (or slip as it's known here) algorithm is obviously proprietary (and thus why difficult to get hold of) but I've never heard that it was adaptive depending on external factors. I did come across a document before which, from memory, gave an overview of the algorithm - that is the multiple delta's from the pulses were effectively 'plotted' and if a preset percentage of points deviated by another preset amount then the whole measurement was voided.

As you will know the accuracy of the device will far exceed +/- 1mph (or equivalent rounded km/h) but one would guess for simplicity that the display only displays integers so with rounding (down) and the possible cosine reduction due to a non-perpendicular measurement then the manufacturer would be a hostage to fortune in listing a more accurate headline accuracy.

In the UK we have quite a generous threshold before enforcement anyway. Such accuracy makes no difference whatsoever in the real world - I don't see any lines of 'attack' here.

I don't know how Canadian law works but here the devices are 'Type Approved' and the measurement is assumed correct unless the defendant can show otherwise - which in reality is near impossible. (But of course the reading is more likely than not to be correct). Additionally, the enforcement officer should be using the device to corroborate their opinion - in most circumstances it's easy to spot a speeding motorist.

The 'anti-sweeping' (or slip as it's known here) algorithm is obviously proprietary (and thus why difficult to get hold of) but I've never heard that it was adaptive depending on external factors. I did come across a document before which, from memory, gave an overview of the algorithm - that is the multiple delta's from the pulses were effectively 'plotted' and if a preset percentage of points deviated by another preset amount then the whole measurement was voided.

Here, police speedometer is assumed to by a prima facie proof if it has been checked to work properly and if the policeman is qualified. In theory, you can counter that by saying that you have checked your car speedometer or that your car could not go to this speed. IN that case I could help. In practice, however, this is very dependant of the judge feeling. Nevertheless, in a previous case, I have found a non documented failure mode of the radar. In consequence, the case was closed outside the court to not create any new jurisprudence.

Anyway, I am VERY interested in any document describing how the algorithm work. The patent of LTI describe an adaptive window methodology to find the peak. This indicates that in less permissive environment the error bar of the measurement will increase. The lidar must use the first «strong» return to trig the clock. The less sharp is this return the larger are the error.

I know it must pass a NTSA test, but this is done in a lab environment. In real life, measurement are much noisier. If the anti-slip algo is statistical it will be much more prone to error that in the lab where it is qualified. I am diffing trough all previous brought to court to understand the specific failure more of lidar.

By the way, does any one know the description fo the pulse train of this radar. I am under the impression there is one pulse, 100 ms wait, then a train of pulse each 8 ms for 0.3 s. It this is right?

QUOTE (justforthepictures @ Thu, 21 Jan 2016 - 11:12)

QUOTE (Yvan Dutil @ Wed, 20 Jan 2016 - 21:10)

I will try to build a full model of the instrument including atmospheric propagation.

Can I ask what you plan to charge your client for such an exercise?

The answer is yes, but not enough! I do charge some time, but it is much less that the true time I am speed on the case. Hopefully, overtime I will be able to reuse my knowledge and get some payback on my effort.

Also, I need to do some technical analysis just to stay mentally in shape. I have no official job right now, and this work help me to keep my skill. Simple differential (Vis vs IR) refraction analysis is not that hard to do. I would like to do the phase screen also, but I must find the adequate scientific literature reference.

By the way, does anyone in tried to used the video as an alternate speed measure? This should by a straight forward prima facie proof.

By the way, does anyone in tried to used the video as an alternate speed measure? This should by a straight forward prima facie proof.

Video, using points of reference with a simple time over distance measurement could be employed - either by the defence or prosecution. However, the person doing this would have to meet certain criteria to be considered an 'expert witness'. But bear in mind such a measurement would be a mean and not an 'instantaneous' measurement like the laser.

By the way, does anyone in tried to used the video as an alternate speed measure? This should by a straight forward prima facie proof.

Video, using points of reference with a simple time over distance measurement could be employed - either by the defence or prosecution. However, the person doing this would have to meet certain criteria to be considered an 'expert witness'. But bear in mind such a measurement would be a mean and not an 'instantaneous' measurement like the laser.

Well, there is open source video analysis software used for physics teaching that can provide frame by frame speed estimate.

Out of interest, is this a speeding case? If so, what was the alleged speed and limit?

179 km/h in a 100 km/h zone. Clocked at 262 m and it is a motorcycle. The police dis a very large sweep to catch the motorcycle. The target is small, far and there is a large sweep. Everything is place for a slip error.

About that! There was two motorcycles. He claim that he has seen the police officer and slowed down while the other guy escaped. There was actually another moto that has escaped. The guy seams honest ans a confusion is possible but this is hard to demonstrate. It might be a lidar failure also. Anyway, legal contestation of speeding ticket is disappearing in Quebec. There is roughly 20 cases per year for over 600 000 tickets and roughly 1700 extreme speeding. Ticket number have been cut by a factor 3 in 5 years due to an increase repression. Probability of winning is roughly 10%. Just to say, I am not making a living from this.

He claim that he has seen the police officer and slowed down while the other guy escaped.

As you know the speed measurement is usually made by the time the officer was spotted and deceleration occurred... Although I agree a bike is a harder target with a very different profile to a car.

Well, in that specific case my client said he had seen the police officer when he was nearing him. The police officer has taken is measurement when I was moving away. It was the noise of the motorcycle that has attracted is attention as he was doing some radar in the opposite direction. This is why there is a large sweep and that the pointing might now have been as stable as in a normal situation.

Off course, nothing is black and white. Nevertheless, I never take a case when I don't think it might have a technical issue involved. Also, my client are warned from the beginning that the physics is useless if they lied.

He claim that he has seen the police officer and slowed down while the other guy escaped.

As you know the speed measurement is usually made by the time the officer was spotted and deceleration occurred... Although I agree a bike is a harder target with a very different profile to a car.

Well, in that specific case my client said he had seen the police officer when he was nearing him. The police officer has taken is measurement when I was moving away. It was the noise of the motorcycle that has attracted is attention as he was doing some radar in the opposite direction. This is why there is a large sweep and that the pointing might now have been as stable as in a normal situation.

Off course, nothing is black and white. Nevertheless, I never take a case when I don't think it might have a technical issue involved. Also, my client are warned from the beginning that the physics is useless if they lied.

I think the pysics will be useless if the physicist is confused about the technologies involved. Is it radar or laser?

If you don't already know anything about the laser device, it is clear you know very little about it, the court is unlikely to consider you an expert in these devices. You may be encouraged to know that the court will at least listen to you but do not be too surprised if they discard your evidence. They are likley to do that if you reference some handy and free garbage you picked up online.

An expert witness for a laser speed gun will be aware of how it manages the physics before that witness examines and comments on the case. Even someone who has a very good knowledge of physics is unlikely to be an expert in laser speed guns. Your questions about the physics involved shows your client is not being well served, indeed and expert witness trawling web forums for tuition in his expert subject, oh dear.